What do you know about keratosis pilaris? Typified by clusters of small bumps in the skin, usually on the backs of the upper arms, thighs, or bottom, this is a condition that affects as many as 40 percent of adults.Keratosis pilaris is caused by keratin building up in your hair follicle, forming a plug of keratiny stuff that looks on the surface like mild acne. Nobody seems to know why it happens. Unfortunately, KP can be exacerbated when your skin is dry, so when you treat it like acne by rubbing it with salicylic acid and the like, you can actually make it worse.